1. Field of the Invention
The present invention relates to an apparatus which performs, by OCDMA (Optical Code Division Multiplex Access), at least one of encoding and decoding of wavelength-division-multiplexed light. More particularly, the present invention relates to an apparatus which employs fiber gratings to perform encoding/decoding by OCDMA.
2. Description of the Related Art
In OCDMA, a technique similar to the CDMA technology which has been practically used in the field of mobile communications is employed to perform encoding of an optical signal at a transmitting end, and decoding of an optical signal at a receiving end. The encoding/decoding of an optical signal is performed by using optical devices such as diffraction gratings, optical waveguides, or fiber gratings.
In OCDMA, even if a number of encoded optical signals exist in the same wavelength band, interferences therebetween are prevented because of code-by-code independence. Therefore, by assigning different codes to different users, it becomes possible for a large number of users to simultaneously share one optical signal propagating medium, even though optical signals in the same wavelength band are used.
Currently proposed encoding methods can be classified into, for example: Frequency-encoding techniques; Frequency-Hopping techniques; Fast-Frequency-Hopping techniques; and direct-sequence techniques. A Frequency-encoding technique is a method of encoding which varies the intensities of optical signals for different wavelengths. A Frequency-Hopping technique and a Fast-Frequency-Hopping technique are methods of encoding which vary wavelength and delay. A direct-sequence technique is a method of encoding which varies delay and phase for a single wavelength.
In “Passive Optical Fast Frequency-Hop CDMA Communications System”, Habib Fathallah, Journal of Lightwave Technology, Vol. 17, No. 3, March 1999, there is proposed a Fast-Frequency-Hopping technique (hereinafter abbreviated as “FFH technique”) which is performed by using fiber gratings which are assigned with different delays corresponding to different wavelengths. The present invention relates to this optical encoding method. This optical encoding method may sometimes be referred to as “time-spread/wavelength-hop optical CDMA”.
First, with reference to FIG. 1, encoding/decoding by the conventional FFH technique will be described.
FIG. 1 corresponds to FIG. 1(b) in Habib. The apparatus shown in FIG. 1 is an encoder, which includes a series connection of optical fibers. Each optical fiber has a uniform fiber grating structure. The optical fibers are of the same structure, but different tensions are applied to the respective optical fibers by utilizing piezoelectric devices.
Since the grating period of each optical fiber varies in accordance with the tension applied thereto, the wavelength band in which Bragg reflection occurs is shifted from optical fiber to optical fiber. Therefore, each wavelength component contained in an incoming optical signal (a broadband light pulse) is reflected by a fiber grating in a different position, depending on the wavelength. Different reflection positions result in different amounts of time being required for the optical signal to make back and forth trips. As a result, the respective wavelength components of the optical signal are output from the optical fibers at different points in time. In other words, if a single broadband pulse is input to the encoder of FIG. 1, a plurality of light pulses will be output at different points in time, depending on their wavelengths.
Now, assume that N optical fiber gratings F1 to FN are employed, with tensions being applied to the optical fiber gratings F1 to FN so that they reflect light at central wavelengths λ1 to λN, respectively. In this case, the reflection central wavelengths λ1 to λN may be of the following order of magnitude (ascending from left to right), for example:λ1<λ2<λ3<, . . . , <λN-1<λN 
In this exemplary case, the wavelength λ1 of light to be reflected by the optical fiber grating F1 is the shortest, while the wavelength λN of light to be reflected by the optical fiber grating FN is the longest. Such an order of reflection wavelengths can be easily changed by changing the combination of tensions to be applied to the N optical fibers F1 to FN. The number of possible permutations is N!=N×(N−1)×(N−2)× . . . 3×2×1. However, among these possible permutations, there may be some which are difficult to be distinguished from one another. Therefore, in actuality, the number of codes (described later) will be smaller than N!.
In the encoder of FIG. 1, optical signal encoding is performed by utilizing such an array of reflection wavelengths. In other words, in the aforementioned example, light which is reflected by the optical fiber F1 follows the shortest optical path before coming out at an input/output section of the optical fibers, and hence sustains the shortest delay.
Thus, in the encoder of FIG. 1, a particular combination of wavelength dependences of delay may be selected from among the N! permutations, and programmed to the encoder as its code pattern.
On the other hand, the apparatus of FIG. 1 may be allowed to function as a decoder. Specifically, by applying appropriate tensions to the optical fibers in the apparatus of FIG. 1, it is possible to cause inverse delays for canceling the delays which have occurred through the above-described encoding. By applying such delays, the encoded optical signal (which comprises a sequence of optical signals having different delays) can be decoded back to the original optical signal in the apparatus shown in FIG. 1.
Next, with reference to FIG. 2, a code pattern which is adoptable in OCDMA will be described.
In FIG. 2, (a) shows a relationship between the wavelength of an optical signal and the delay to be applied by an encoder, with respect to a particular code (hereinafter, such a relationship may be referred to as a “delay pattern”). In FIG. 2, (b) shows a relationship between the wavelength of an optical signal and the delay to be applied by a decoder for decoding an optical signal which has been encoded so as to have the delay pattern shown in (a) of FIG. 2. As seen from (a) and (b) of FIG. 2, an encoder and its corresponding decoder are supposed to have opposite delay patterns.
When the apparatus shown in FIG. 1 is to be employed in the context of wavelength division multiplexing (WDM), it becomes necessary to provide as many apparatuses of FIG. 1 as there are divided wavelength bands. This point will be described below, with reference to FIG. 3.
FIG. 3 schematically shows three codes (Code 1, Code 2, Code 3) against the central wavelengths λ1 to λ4 of four divided wavelength bands (wavelength channels). When OCDMA is used in conjunction with WDM, it becomes possible to assign different codes to each channel. As a result, a single optical signal propagating medium can be effectively shared by a large number of users for performing communications.
However, when OCDMA is to be used in conjunction with WDM, as many OCDMA encoder/decoders will be required as there are wavelength channels. As the number of WDM wavelength channels increases beyond ten, and even twenty in the future, a substantial increase in the encoder/decoder size would be inevitable.